Cryptography Symmetric Encryption

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Cryptography Symmetric Encryption Week 9 Stallings: Ch 3 & 6 Stallings: Ch 4 CNT-4403: 19.March.2015 1 Today’s Class . Symmetric Ciphers . Multiple Encryption . Modes of Operation CNT-4403: 19.March.2015 2 Symmetric Cryptosystems Encryption Key Decryption Key Plaintext Plaintext Encryption Decryption Algorithm = Algorithm CNT-4403: 19.March.2015 3 Symmetric Ciphers . Stream Ciphers . Block Ciphers CNT-4403: 19.March.2015 4 Stream Ciphers . Encrypt one bit (byte) at a time . Example: Vigenere, Vernam . Length of key = length of (clear/cipher) text . Hard to share between sender and receiver Bit-stream Bit-stream Key (K) Key (K) generator generator Key ki Key ki Plaintext Ciphertext Plaintext Pi Ci Pi Plaintext Plaintext CNT-4403: 19.March.2015 5 Block Ciphers . Encrypt one block of text at a time . 64-128 bit long . Encryption key = Decryption key Focus: Block Ciphers! . Shared by sender and receiver 64 bits 64 bits Key (K) Encryption Algorithm Plaintext Ciphertext CNT-4403: 19.March.2015 6 Block Cipher Principles . n bit input to n bit output . 2n possible inputs . Each must produce a unique cipertext . Otherwise encryption is not reversible . No decryption possible CNT-4403: 19.March.2015 7 Ideal Block Cipher Need 2n table to encrypt! CNT-4403: 19.March.2015 8 Feistel Cipher . Introduced by Horst Feistel . 16 + 1 rounds Plaintext What is F ? LE0 RE0 F K1 Li = Ri–1 LE1=RE0 RE1 Ri = Li–1 F(Ri–1, Ki) CNT-4403: 19.March.2015 9 Feistel Cipher Structure CNT-4403: 19.March.2015 10 Data Encryption Standard (DES) . Most widely used block cipher in world . Adopted in 1977 by NBS (now NIST) . As FIPS PUB 46 . Encrypts 64-bit data using 56-bit key . Has been considerable controversy over its security CNT-4403: 19.March.2015 11 DES History . IBM developed Lucifer cipher . Team led by Feistel in late 60’s . Used 64-bit data blocks with 128-bit key . Redeveloped as a commercial cipher with input from NSA and others . 1973: National Bureau of Standards (NBS) issued request for proposals for a national cipher standard . IBM submitted their revised Lucifer which was eventually accepted as the DES CNT-4403: 19.March.2015 12 DES Controversy . DES standard is public . Considerable controversy over design . Choice of 56-bit key (vs Lucifer 128-bit) . Design criteria were classified . Subsequent events and public analysis show in fact design was appropriate . Use of DES has flourished . Especially in financial applications . Still standardised for legacy application use . To be replaced by AES CNT-4403: 19.March.2015 13 DES Encryption CNT-4403: 19.March.2015 14 Initial Permutation (IP) . First step of the data computation . IP reorders the input data bits . Even bits to LH half, odd bits to RH half . Quite regular in structure (easy in h/w) . Example: IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb) CNT-4403: 19.March.2015 15 DES “Round” Structure . Uses two 32-bit L & R halves . Feistel cipher: Li = Ri–1 Ri = Li–1 F(Ri–1, Ki) CNT-4403: 19.March.2015 16 DES Structure: Function F F takes 32-bit R half and 48-bit subkey: . Expands R to 48-bits using perm E . Adds to subkey using XOR . 8 S-boxes to get 32-bit result . Finally permutes using 32-bit perm P CNT-4403: 19.March.2015 17 .

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Block Ciphers and the Data Encryption Standard
Lecture 3: Block Ciphers and the Data Encryption Standard Lecture Notes on “Computer and Network Security” by Avi Kak ([email protected]) January 26, 2021 3:43pm ©2021 Avinash Kak, Purdue University Goals: To introduce the notion of a block cipher in the modern context. To talk about the infeasibility of ideal block ciphers To introduce the notion of the Feistel Cipher Structure To go over DES, the Data Encryption Standard To illustrate important DES steps with Python and Perl code CONTENTS Section Title Page 3.1 Ideal Block Cipher 3 3.1.1 Size of the Encryption Key for the Ideal Block Cipher 6 3.2 The Feistel Structure for Block Ciphers 7 3.2.1 Mathematical Description of Each Round in the 10 Feistel Structure 3.2.2 Decryption in Ciphers Based on the Feistel Structure 12 3.3 DES: The Data Encryption Standard 16 3.3.1 One Round of Processing in DES 18 3.3.2 The S-Box for the Substitution Step in Each Round 22 3.3.3 The Substitution Tables 26 3.3.4 The P-Box Permutation in the Feistel Function 33 3.3.5 The DES Key Schedule: Generating the Round Keys 35 3.3.6 Initial Permutation of the Encryption Key 38 3.3.7 Contraction-Permutation that Generates the 48-Bit 42 Round Key from the 56-Bit Key 3.4 What Makes DES a Strong Cipher (to the 46 Extent It is a Strong Cipher) 3.5 Homework Problems 48 2 Computer and Network Security by Avi Kak Lecture 3 Back to TOC 3.1 IDEAL BLOCK CIPHER In a modern block cipher (but still using a classical encryption method), we replace a block of N bits from the plaintext with a block of N bits from the ciphertext.
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Feistel Cipher
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